for ($i = self::ROUNDS; $i > 0; $i -= 2) {
            $x4  = $x4->xorInt32($x0->addInt32($x12)->rotateLeft(7));
            $x8  = $x8->xorInt32($x4->addInt32($x0)->rotateLeft(9));
            $x12 = $x12->xorInt32($x8->addInt32($x4)->rotateLeft(13));
            $x0  = $x0->xorInt32($x12->addInt32($x8)->rotateLeft(18));